This invention pertains to sliding-vane pumps, specifically to a wear indicator for their vanes.
Rotary devices, such as vacuum and pressure pumps, contain components whose wear rate directly influences the reliable, useful operating fife of the entire device. These components can be referred to as critical components. Additionally, continued usage of the device beyond the expected useful life of a critical component often results in catastrophic, sudden and complete, failure of the device and the entire system of which it is a part, without any significant preceding degradation of system performance. This catastrophic failure, without warning, is an especially troublesome characteristic of vacuum and pressure pumps used to drive the gyroscopic flight instruments installed in light aircraft. Malfunction of such instruments while flying in instrument meteorological conditions can lead to loss of aircraft control with disastrous results.
Substantially all of the sliding vane pump failures occur because the vanes become worn to the point where their length of engagement in the rotor slot is too short to provide adequate support. The vane, typically made of graphite, breaks and subsequently causes the remaining vanes to fail. Laboratory testing has been done to determine the maximum wear limit of the vanes beyond which pump failure, within a relatively short time, is a certainty.
Prior relevant developments, designed to determine the state of wear of the vanes, include a removable plug in the pump housing to enable viewing of the rotor and vanes to determine their remaining length. In one of those developments, a plastic go-no-go stick is inserted into the opening. In another development, the aircraft mechanic compares the relative position of two reference holes, one each in the vane and rotor.
Prior developments suffer from a number of disadvantages:
a) The prior developments are approximate, at best, since wear rates of the vanes vary from vane to vane and from pump to pump and during the life of the pump. Laboratory tests show that an error in measurement of vane length of a few thousandths of an inch represents the amount of wear that a typical light aircraft vacuum pump would experience in an entire year.
b) The steps required by both of the above mentioned prior developments are considered to be maintenance by the FAA, and as such, must be conducted by an appropriately certified mechanic. An owner of an aircraft, who is not a certified mechanic, is not permitted to conduct the above mentioned inspection procedures unless he does so while under the supervision of an FAA certified mechanic. This FAA requirement represents an inconvenience and added expense for the aircraft owner.
c) Rather than permitting the pilot to determine if the pump is near its service life limit, at a frequency and time of his choosing, existing methods of determining the state of wear of the vanes are snapshot measurements taken at prescribed intervals.
d) Prior developments require multiple internal inspections to estimate when the pump should be replaced or rebuilt.
The automatic wear indicator described in this patent application provides for the encapsulation of tracer material within the vanes of sliding vane pumps in such a manner that it is automatically released into the discharge conduit at the precise time that the vane reaches a safe wear limit. Presence therein signals that the pump has reached it""s service life.
Accordingly, several objects and advantages of this invention are:
a) to provide a warning of eminent failure
b) to provide an automatic indication of vane wear
c) to provide an accurate indication of vane wear
d) to eliminate the need to employ the services of a certified mechanic
e) to permit the pilot to determine if the vanes have reached their service limit
f) to provide for automatic switching on of annunciators and other devices
g) to eliminate the need to disassemble or remove any components of the pump for internal inspection.